For numerous applications it is desirable to have an apparatus and method to synchronize a process to the heartbeat of a human or other animal. A patient monitor or other equipment may monitor an electrocardiogram (ECG) and by use of electronic circuitry and a software signal processing algorithm produce a synchronization pulse, also called an R-wave trigger or an R-wave gate, that can be used by integral circuitry or by additional externally attached equipment.
One application is cardiovascular where the heart has upper chambers, the atria, which are in a disorganized rhythm of contraction known as Atrial Fibrillation (AFIB). Lower chambers, the ventricles, do have an organized rhythm of contraction. In this case a shock delivered at the proper moment can reorganize the atria and terminate this dangerous heart rhythm. Because the lower chambers, the ventricles, are the major pumping muscles and in this AFIB rhythm the ventricles are in an organized pumping rhythm, AFIB is not immediately dangerous. For the cardioversion process, however, if the shock is delivered at an improper moment, the ventricles are vulnerable to converting to a disorganized contraction known as Ventricular Fibrillation (VFIB) which is immediately dangerous since all pumping action ceases. Permanent brain damage occurs within a few minutes. Therefore it is essential to deliver the cardioversion shock at the most proper and safe moment. Current device standards understand and guide the device manufacturer on this issue. Cardioversion shock should be synchronized to the period of the R wave of the ECG and not later than 30 milliseconds after the peak of the R wave.
Another application is image acquisition. Images acquired by machines like Computerized Tomography (CT) scanners or Fluoroscope scanners or ultrasound benefit from combining images collected from a consistent moment in the cycle of numerous heartbeats. An example is the moment in each heartbeat where the muscle is completely relaxed, diastole. If a trigger is available at the instant of each R-wave, the imaging equipment can take a series of images at a fixed delay. Combining multiple images can provide a much higher quality image where random noise has been significantly reduced.
Another application example is non-invasive blood pressure measurement. Here a device such as a cuff and microphone are components in a system that observes the acoustic response (Korotkoff sounds) of an artery in the arm as pressure in a cuff is reduced. At each heartbeat when the cuff pressure is in a certain range some blood can squirt through the artery and make the sound. A detector has a much better confidence of detecting these sounds if an independent trigger is available to indicate the moment of each heartbeat.
For many applications, but not all, a trigger is desired only for the normal heartbeats. The heart can generate spontaneously abnormal heartbeats, such as Premature Ventricular Contractions (PVC) or Premature Atrial Contractions (PAC) and several others. These abnormal heartbeats, also called ectopic heartbeats, are frequently ineffective or less effective at pumping blood. The path of cell to cell depolarization of the heart muscle is usually different for these beats and so the actual mechanical motion of the heart will be different. For most imaging techniques it is desirable that no image be added to the ensemble average for these abnormal beats. Also for the blood pressure application, R wave gates on abnormal beats are not helpful since blood pressure production from these beats is significantly different from normal beats.
An important clarification is that the component of the ECG waveform known to signify the electrical depolarization and the mechanical contraction of the main heart muscles is called the QRS component. The R-wave is generally understood to be the largest positive part of the QRS. For triggering systems, if the largest part of the QRS is a negative wave, technically a Q or an S wave, it is desirable to trigger on the negative wave. Such a system is still referred to as a R-wave trigger or gate. Frequently the terms R-wave or QRS are used interchangeably.
Conventional R-wave triggers suffer from a sensitivity to noise in the ECG, or lack of consistency in the time relationship of the trigger to the R-wave, and/or lack of specificity to normal heartbeats. More sophisticated arrhythmia analysis algorithms do reliably detect heartbeats and usefully classify beats as normal or abnormal, but these more sophisticated algorithms generally require substantially more computer power (more central processing unit (CPU) cycles per second and more memory usage for the program and data) and generally have a significant delay in determination; not less than 200 milliseconds and more typically one to two seconds delay before classification is known.
Therefore, there is a need to have a method and apparatus that can detect and classify an R-wave as normal or abnormal. There is also a need for this detection and classification to occur within about 30 milliseconds. There is also a need for this detection and classification to be as simple and efficient as possible for incorporation in small, low cost, and low power equipment in addition to applications in larger and more expensive equipment.